Field
The present disclosure relates generally to radio-frequency (RF) generators and, more specifically, to apparatuses and techniques for improving the performance of RF generators that employ balanced power amplifiers to supply power to a plasma processing chamber.
Background
Balanced amplifiers, as described for instance by K. Kurokawa, “Design theory of balanced transistor amplifiers,” Bell System Technical Journal, Oct. 1965, are often used in power supply systems for plasma processing chambers. Unlike traditional single amplifiers, balanced amplifiers provide more desirable pulse shapes when using pulsed power to sustain a plasma. Balanced amplifiers also provide better plasma stability and further produce forward power independent of load impedance. In other words, for a given control input to the power amplifier, forward power does not change as a result of load impedance changes (e.g., due to changes in the plasma density or other plasma characteristics).
Yet, balanced amplifiers also create and amplify problems associated with traditional single amplifiers. For instance, when there is a load mismatch (e.g., during ignition or reignition of the plasma, where impedance changes drastically), power dissipation becomes unevenly distributed between the two amplifiers making up the balanced amplifier, which can damage the one dissipating more power. The traditional solution to this problem is to reduce the balanced amplifier's power profile (e.g., reduce load power at high load reflection coefficient magnitude) such that the amplifier dissipating more power is not damaged. In plasma ignition applications, this is a major drawback since plasma ignition normally requires an amplifier delivering substantial power into a non-matched load.
There is, therefore, a need in the art for an improved RF generator to supply power to a plasma processing chamber.